Whilst I agree with most of your examples, I disagree with your email scenario. Why do you feel that having an example email address as a placeholder is considered bad practice?
Placeholders can be useful for providing additional context, however like you, I agree they should not be used to include important instructions. In the email example. if the placeholder did disappear due to contrast issues, focused input, browser not supported or whatever else, I doubt many users would be left confused as to how to proceed with filling out their email address! Placing the example email address below the input is superfluous in my opinion.
I also disagree with leaving the placeholder visible when a user clicks into the input. The reasons are two-fold.
Firstly, the contrast you have used in your example is not strong enough. The placeholder text would need to be darker. A darker placeholder could then cause confusion if placed alongside other fields that have been filled out. Filled-out fields will be harder to differentiate.
Secondly, as I mentioned above, the role of the placeholder is not meant to be a permanent feature, if it was, it would be outside of the field. I strongly doubt that the ‘disappearing placeholder text places a burden on the user’s short-term memory’.